Critical stability conditions in skimmed milk crossflow microfiltration: impact on operating modes

Abstract

There exists a critical ratio (convection towards the membrane/erosion) in crossflow micro- filtration, MF, below which there is no marked fouling by colloidal particles and above which per- formance are altered: sharp increase of fouling, reduced operating time, large decrease in perme- ability and solute transmission. This paper outlines the impact of the critical ratio on skimmed milk MF processing (separation of casein micelles from the soluble proteins) and gives objective ele- ments (taking into account the critical ratio) for the selection of start-up procedures and stationary modes of operation: controlled transmembrane pressure or permeation flux; static or dynamic counter- pressure, the latter achieved by circulating the permeate co-current to the retentate to maintain an equal transmembrane pressure profile along the filtering path. Results showed that the controlled trans- membrane pressure mode of operation was appropriate for conducting skimmed milk filtration, since it prevented from sharp increase of the hydraulic resistance of fouling and maintained nearly constant performances (permeability, selectivity) over the course of the time. In order to determine the more appropriate counter pressure mode and to predict skimmed milk performances, a four step method was proposed : i) assessment of the critical ratio; ii) selection of the critical flux at the required wall shear stress; iii) determination of the corresponding critical transmembrane pressure in dynamic counter-pressure; iv) calculation of the MF performance in static mode taking into account the gradient of transmembrane pressure induced by the retentate pressure drop. hydrodynamics / crossflow microfiltration / operating mode / critical permeation flux / milk